LM5005MH National Semiconductor, LM5005MH Datasheet - Page 13

• Manufacturer Part Number: LM5005MH
• Description: IC, BUCK REGULATOR, TSSOP-20
• Manufacturer: National Semiconductor

Specifications of LM5005MH

• Primary Input Voltage: 75V
• No. Of Outputs: 1
• Output Voltage: 1.225V
• Output Current: 2.5A
• No. Of Pins: 20
• Operating Temperature Range: -40°C To +125°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Current Limit
The LM5005 contains a unique current monitoring scheme for
control and over-current protection. When set correctly, the
emulated current sense signal provides a signal which is pro-
portional to the buck switch current with a scale factor of 0.5
V / A. The emulated ramp signal is applied to the current limit
comparator. If the emulated ramp signal exceeds 1.75V
(3.5A) the present cycle is terminated (cycle-by-cycle current
limiting). In applications with small output inductance and high
input voltage the switch current may overshoot due to the
propagation delay of the current limit comparator. If an over-
shoot should occur, the diode current sampling circuit will
detect the excess inductor current during the off-time of the
buck switch. If the Sample and Hold DC Level exceeds the
1.75V current limit threshold, the buck switch will be disabled
and skip pulses until the diode current sampling circuit detects
the inductor current has decayed below the current limit
threshold. This approach prevents current runaway condi-
tions due to propagation delays or inductor saturation since
the inductor current is forced to decay following any current
overshoot.
Soft-Start
The soft-start feature allows the regulator to gradually reach
the initial steady state operating point, thus reducing start-up
stresses and surges. The internal soft-start current source,
set to 10µA, gradually increases the voltage of an external
soft-start capacitor connected to the SS pin. The soft-start
capacitor voltage is connected to the reference input of the
error amplifier. Various sequencing and tracking schemes
can be implemented using external circuits that limit or clamp
the voltage level of the SS pin.
In the event a fault is detected (over-temperature, Vcc UVLO,
SD) the soft-start capacitor will be discharged. When the fault
condition is no longer present a new soft-start sequence will
commence.
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Boost Pin
The LM5005 integrates an N-Channel buck switch and asso-
ciated floating high voltage level shift / gate driver. This gate
driver circuit works in conjunction with an internal diode and
an external bootstrap capacitor. A 0.022µF ceramic capacitor,
connected with short traces between the BST pin and SW pin,
is recommended. During the off time of the buck switch, the
SW pin voltage is approximately - 0.5V and the bootstrap ca-
pacitor is charged from Vcc through the internal bootstrap
diode. When operating with a high PWM duty cycle, the buck
switch will be forced off each cycle for 500ns to ensure that
the bootstrap capacitor is recharged.
Under very light load conditions or when the output voltage is
pre-charged, the SW voltage will not remain low during the
off-time of the buck switch. If the inductor current falls to zero
and the SW pin rises, the bootstrap capacitor will not receive
sufficient voltage to operate the buck switch gate driver. For
these applications, the PRE pin can be connected to the SW
pin to pre-charge the bootstrap capacitor. The internal pre-
charge MOSFET and diode connected between the PRE pin
and PGND turns on each cycle for 250ns just prior to the onset
of a new switching cycle. If the SW pin is at a normal negative
voltage level (continuous conduction mode), then no current
will flow through the pre-charge MOSFET/diode.
Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the
integrated circuit in the event the maximum junction temper-
ature is exceeded. When activated, typically at 165 degrees
Celsius, the controller is forced into a low power reset state,
disabling the output driver and the bias regulator. This feature
is provided to prevent catastrophic failures from accidental
device overheating.
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